1. Field of the Invention
The present invention relates to an optical communication system and a transmission apparatus for use therein, and, more particularly, to an optical communication system suitable for multi-value optical communication in which signal is transmitted by modulating the intensity of light according to a multi-value digital signal and a transmitting apparatus for use in this system.
2. Related Background Art
In recent years, the speed of optical data transmission has become faster, and at the same time the capacity of data transmission has become greater. To achieve these goals, there have been developed, e.g., a high-speed system wherein a signal is transmitted at a higher bit rate, a wavelength division multiplexing system wherein optical signals having different wavelengths are transmitted through a common optical transmission path, and so forth.
Among these developments, a multi-value digital communication system is effective as a method for increasing the capacity of a communication system, compared with a conventional binary digital communication system, as the former can transmit more data than the latter in a communication system having the same bit rate.
In an optical communication system, two kinds of intensity-modulation methods are usually used for modulating light according to a transmission signal, i.e., direct modulation and external modulation.
In direct modulation, a current injected into a semiconductor laser, which is generally used as a light source in the optical communication systems, is controlled to modulate its intensity of light emission. In the external modulation method, a constant current is injected into a semiconductor laser to cause the laser to radiate a beam of a constant intensity, and the intensity of this output from the semiconductor laser is modulated by an external modulator. Samples of the external include a deflection-type light modulator in which the modulation is done by deflecting a laser light using the acousto-optic effect or electro-optic effect, an absorption-type light modulator in which the modulation is conducted by utilizing a wavelength-dependency of light absorption in semiconductors, and an interference-type light modulator in which the modulation is performed by dividing a laser light into two light beams, shifting the phase of one light beam and then combining the two light beams to cause interference between them.
However, where the intensity of an optical signal is modulated in, for example, a multi-value digital optical communication system, the following problems occur.
While the injection current into a semiconductor laser is controlled to perform the optical modulation, as discussed above, where the radiation intensity of a semiconductor laser is to be directly modulated, the relationship between a current injected into a semiconductor laser and its radiation intensity is generally non -linear. Therefore, a sophisticated control of an injection current is required according to the value of a multi-valued transmission signal, and hence a complicated current control circuit is needed. Further, since automatic power control (APC) which is usually preferred cannot be performed in the direct modulation method, the intensity of light radiation will change even for the same amount of an injection current owing to the change of a semiconductor laser with aging. Moreover, when a semiconductor laser is directly modulated, the wavelength of light radiation thereof fluctuates due to the phenomenon called chirping. As a result, there is also a problem that the multiplexing cannot be done with multiple wavelengths being close to one another too much.
In the external modulation method, where the deflection-type light modulation is carried out using the acousto-optic effect or the like, it is difficult to attain a multi-value communication because light modulation due to the control of a deflection angle can only be performed between "deflection" and "non-deflection" or "ON" and "OFF". Further, when the ratio of an amount of deflected light is to be controlled, it is necessary to accurately control, according to the value of a multi-valued signal, the electric power of a radio frequency signal to be applied to an acousto-optic device. As a result, a sophisticated circuit is also needed in this case.
Moreover, when the absorption-type or the interference-type light modulator is used, the amount of light modulation does not change linearly for the amount of a control current or a control voltage, and hence a complicated control is required. In addition thereto, the amount of a control current and the value of a control voltage need to be set or adjusted beforehand. Further, when the relationship between the control current or voltage and the modulation amount varies owing to the change with age, the amount of a transmission light corresponding to each value of a multi-valued signal will change accordingly. As a result, there occurs a problem that the error rate at the time of a signal transmission deteriorates.